Examples of high pressure fuel injection systems are shown in U.S. Pat. No. 4,275,844 issued to Grgurich et al. and U.S. Pat. No. 5,191,867 issued to Glassey et al. on Mar. 9, 1993. Engines equipped with high pressure fuel injection systems have an optimal volumetric injection rate. For diesel-cycle engines, this optimal injection rate has a gradual rise, a period of stabilization, followed by a sharp drop. Means of producing this characteristic profile are commonly referred to as rate shaping means or devices because they are used to shape the volumetric rate of fuel injection into an engine combustion chamber. The gradual rise followed by a sharp drop in fuel injection has the specific benefit of minimizing particulate emissions from combustion. It also minimizes combustion noise.
Fuel injector nozzles typically include a housing with an elongated cavity or void along a first axis. The cavity has a first end portion or injection chamber and a second end portion or spring chamber with a connecting guide passage disposed therebetween. An injection orifice fluidly connects the injection chamber of the cavity with an atmosphere (e.g., engine combustion chamber) external to the fuel injector. A needle check is slidably disposed within the cavity for translation between a first position in which a seat portion of the needle check seats against a first end or bottom of the cavity, the injection orifice and a second position wherein the needle is spaced from the first end and does not block the injection orifice.
In the fuel injector nozzle of Glassey et al., a spring is disposed against the needle check which tends to bias the needle toward the first end. The spring chamber of the cavity has an opening providing fluid communication with a low pressure fuel supply. Pressurized fuel directed to the injection chamber of the cavity overcomes the spring to move the check away from the first end. Any fluid in the spring chamber of the cavity displaced by movement of the check theretoward is exhausted through the opening connecting to the low pressure fuel supply.
The fuel injector nozzle disclosed by Grgurich does not have a fluid communication opening in the spring chamber. During an injection cycle, fluid seeps past the guide portion of the needle check from the high pressure injection chamber to the spring chamber, increasing the pressure within the spring chamber. The increase in pressure in the spring chamber of the cavity increases the valve opening pressure (VOP) of fluid in the injection chamber needed to lift the check from the first end of the cavity. Too high of a VOP produces a very steep initial rate of fuel injection which has the undesirable effect of increasing engine combustion noise and increasing nitrogen oxides (NO.sub.x).
It is desired to provide a fuel injector nozzle having a relatively low VOP and providing a gradually rising volumetric rate of injection with a crisp end of injection to provide a low valve opening pressure, and to minimize engine combustion noise and NO.sub.x.